1. Field of the Invention
The present invention relates to a chemical mechanical polishing (referred to as CMP hereafter) device; more specifically, the invention relates to a CMP device with a pressure-controlling mechanism for planarizing silicon wafers.
2. Description of Related Art
Conventionally, the CMP process has been relied upon heavily for providing a complete planarization process to each of the silicon wafers in the production of ULSI devices. An example of such conventional CMP device is illustrated in FIG. 1A.
The above-mentioned conventional CMP device comprises at least the following components: an automated rotating polishing plate 110 having a rotating plate 100 and a polishing pad 120, wherein the main function of the rotating plate 100 is to support and rotate the polishing pad 120; a slurry supplying system 130 is provided for supplying slurry 150 to a surface of a polishing pad 120; and a rotating carrier 160 having a spindle 180 for holding and rotating a silicon wafer 140 that is to be polished by the polishing pad 120 and slurry 150 during a CMP process.
Furthermore, a conventional CMP device typically comprises a rotating polishing plate 110 and a rotating carrier, each rotating independently while exerting a pressure force P to opposite sides of the wafer. The slurry used in a CMP process is typically comprised of silica or alumina particles dispersed and suspended in a gel-like acidic or basic etching solution of KOH or NH.sub.4 OH. Then an automated slurry supplying system 130 supplies slurry 150 to the polishing pad 120 in order to maintain a constant and uniform permeation of the slurry 150 on the polishing pad 120.
The mechanisms involved in the CMP process depend heavily on a chemical polishing, wherein the etching solution in the slurry 150 chemically removes or modifies surface particles of a silicon wafer, while a mechanical polishing of the silicon wafer 140 is achieved through the suspended abrasive particles in the slurry 150 and the rotating action of the polishing pad 120. In addition, waste particles produced on the wafer 140 surface during the chemical polishing are also mechanically removed. Therefore, the overall polishing rate for the wafers can be accelerated by increasing either the chemical or the mechanical polishing rate.
It has always been a goal with conventional CMP devices or machines to polish the entire surface of a silicon wafer 140 in a uniform fashion. The contributing factors that directly affect the wafer polishing rate include the intensity and distribution of pressure force exerted to the wafer surface, relative velocities among each point of location on the wafer surface to the rotating polishing plate 110, properties intrinsic to the compositions of the polishing pad and slurry, and complexity of the ULSI circuit layouts formed on the wafer 140.
Shown in FIG. 1B, as the silicon wafer 140 is pressed against the polishing pad 120, the supposedly flat surface of the polishing pad 120 tends to be deformed due to uneven pressures distributed to the surface of the silicon wafer 140; specifically, there are four locations on the surface of the polishing pad 120, namely We, W.sub.e, W.sub.e 1, W.sub.c, and W.sub.en, where the measured contact pressures are the most distinct. Each of the locations, or referred to as contact locations hereafter, has a ring shape which is concentric to all the other contact locations. In particular, the contact location We represents a location on the polishing pad 120 which is in direct contact with the edge of the silicon wafer 140. W.sub.e 1, on the other hand, represents a contact location on the polishing pad 120 next to W.sub.e which in not in direct contact with the silicon wafer 140. W.sub.c represents a contact location on the polishing pad 120 which is in direct contact with the center of the silicon wafer 140, and W.sub.en represents a contact location on the silicon wafer 140 which is situated between W.sub.e 1 and W.sub.c. Furthermore, since the contact pressure P exerted by the polishing pad 120 to the silicon wafer 140 at the edge location W.sub.e is the greatest and the contact pressure P at the contact location W.sub.e 1 is the least, an uneven distribution of the contacting pressure is thus unfavorably created as shown in FIG. 1C. This is then a factor for creating instability.
In addition, the mechanical polishing rate increases as the contacting pressure is increased and vice versa, which in turn generates an unstable physical profile W.sub.s of the wafer at the above-mentioned contacting and non-contacting positions as indicated by FIG. 1D. When peaks and troughs appear in the profile W.sub.s of a wafer as a result of an uneven wafer polishing, waste particles tends to be accumulated on the wafer surface at the position corresponding to W.sub.e 1 while the wafer surface at the position corresponding to W.sub.e tends to be over-polished.